1. Field of the Invention
The present invention relates to a novel organometallic complex that is capable of converting a triplet excited state into luminescence. In addition, the present invention relates to a light-emitting element, a light-emitting device, an electronic device, and a lighting device each using the organometallic complex.
2. Description of the Related Art
In recent years, a light-emitting element using a light-emitting organic compound or an inorganic compound as a light-emitting material has been actively developed. In particular, a light-emitting element called an EL (electroluminescence) element has attracted attention as a next-generation flat panel display element because it has a simple structure in which a light-emitting layer containing a light-emitting material is provided between electrodes, and characteristics such as feasibility of being thinner and more lightweight and responsive to input signals and capability of driving with direct current at a low voltage. In addition, a display using such a light-emitting element has a feature that it is excellent in contrast and image quality, and has a wide viewing angle. Further, since such a light-emitting element is a plane light source, the light-emitting element is considered applicable to a light source such as a backlight of a liquid crystal display and lighting.
In the case where the light-emitting substance is an organic compound having a light-emitting property, the emission mechanism of the light-emitting element is a carrier-injection type. That is, by applying a voltage with a light-emitting layer provided between electrodes, electrons and holes injected from electrodes recombine to put the light-emitting substance into an excited state, and light is emitted when the excited state returns to a ground state. There are two types of the excited states which are possible: a singlet excited state (S*) and a triplet excited state (T*). In addition, the statistical generation ratio thereof in a light-emitting element is considered to be an S*-to-T* ratio of 1:3.
In general, the ground state of a light-emitting organic compound is a singlet state. Light emission from a singlet excited state (S*) is referred to as fluorescence where electron transition occurs between the same multiplicities. In contrast, light emission from a triplet excited state (T*) is referred to as phosphorescence where electron transition occurs between different multiplicities. Thus, in a compound emitting fluorescence (hereinafter referred to as fluorescent compound), in general, phosphorescence is not observed at room temperature, and only fluorescence is observed. Accordingly, the internal quantum efficiency (the ratio of generated photons to injected carriers) in a light-emitting element using a fluorescent compound is assumed to have a theoretical limit of 25% based on the S*-to-T* ratio of 1:3.
In contrast, internal quantum efficiency in a light-emitting element using a compound emitting phosphorescence (hereinafter referred to as phosphorescent compound) can be 100% in theory when light emission led from intersystem crossing from a singlet excited state to a triplet excited state is taken into account. For these reasons, the light-emitting element using a phosphorescent compound has been actively developed in recent years in order to provide a highly efficient light-emitting element.
As a phosphorescent compound, an organometallic complex having iridium as a central metal has attracted attention. Development has enabled phosphorescent compounds to emit light with various wavelengths from red to blue; however, phosphorescence is the light emission from a triplet level which is lower than a singlet excited level in terms of energy, and a phosphorescent compound having an extremely wide energy gap is necessary for obtaining green to blue phosphorescence with large energy. Such substances are difficult to develop and the variety thereof is still narrow.
Patent Document 1 discloses, as a phosphorescent compound emitting phosphorescence with a short wavelength, an iridium complex in which an imidazole derivative is a ligand.